Let's analize triangles ADB and BDC:
- angle DAB = angle BCD
- BD is a common lature
- AB = BC = 5
Rezults from the case of congruence side-side-angle triangle ADB= triangle BDC so angleADB=angleBDC
AB is horizontal and CD is vertical, so if we extend the side AB we'll have a right angle where AB intersect CD (point F). In triangle AFD, F=90 degrees, A=40 degrees, so angleADF = angleADC = 180-90-40 = 50 degrees.
AngleADB is half of angleADC, therefore is 25 degrees.